1. Field of the Invention
The present invention relates to means for controlling the starters of turbine plants.
2. Discussion of the Background
Air-operated starters of turbine plants, particularly turbojet engines, comprise a turbine driven by an auxiliary compressed air generator, a reduction gear and a clutch system. The clutch system may take one of two forms, being either of the "free wheel" type (toothed wheel and detent), or of the "inertia launcher" type (claws or dog-teeth), and in each case the starter is operated by opening a butterfly valve in the compressed air supply duct to feed the starter turbine.
However, according to the type of clutch used in the starter, the characteristics required for the opening of the starter valve are conflicting. The free wheel system requires acceleration of the starter to remain within specific limits when the starter valve is opened, in order to minimize impact on the clutch, particularly when the engine is already turning over (auto rotation). On the other hand, the inertia system requires an acceleration rate greater than a certain minimum value in order positively to engage the claw-engagement system between the starter and the engine.
Now, as starters are manufactured by different equipment manufacturers and may be mounted equally on a particular engine according to the customer's choice, it would be advantageous for the engine manufacturers to provide starter valve systems capable of being used with both the above-mentioned types of starter.
However, conventional starter valve systems are quite inadequate to deal with the essential, but contrasting, conditions mentioned earlier, and make it difficult to use a system which is common to both types of starter.
Indeed, conventional starter valve systems employ an effective restriction in a supply to or discharge from an actuator controlling the butterfly valve. As a result, during the first few degrees of opening, the slight gain in "cross-section/angle of opening" of the butterfly valve means that the rate of pressure increase at the inlet to the starter turbine remains low, leading to poor acceleration of the starter. If this working restriction is eliminated so as to permit satisfactory functioning of inertia launcher starters, the effect on a free wheel starter would be to exceed the impact capacity of the clutch due to the excessive acceleration which it would induce in the reduction gear in the case of auto-rotation.
It must also be noted that these contrasting characteristics are aggravated by the fact that the supply pressure from the auxiliary compressed air generator upstream of the starter valve may vary within a ratio of 3:1, as shown in FIG. 2 which reproduces the characteristic curves of supply pressure increase as a function of time with respect to conventional starter valve systems.